The charging of a lithium-ion battery begins with a stage of charging at constant current that is followed by a stage of charging at constant voltage once the sum of the back-electromotive force (back-emf) of the battery plus the voltage drop caused by the internal resistance of the battery is equal to the output voltage from the charger. Since it is presently recommended to charge the battery at a maximum voltage of 4.2 volts per storage cell present in the battery, the output voltage of the charger is set on that value.
In cold weather, the internal resistance of the battery is very high, such that the stage of charging at constant current is short. Unfortunately, since the stage of charging at constant current is the most efficient stage, that means that the time taken to charge the battery is itself lengthened in a manner corresponding to the extent to which said stage is shortened.